Electric discharge- The loss of electricity by any electrified body, i.e., the radiation of this body, can occur in various ways, as a result of which the phenomena accompanying the radiation can be very different in nature. All the various forms of R. can be divided into three main types: R. in the form of electric current, or R. conductive, R. convective and R. discontinuous. R. in the form of current occurs when an electrified body is connected to the earth or to another body possessing electricity equal in quantity and opposite in sign to the electricity on the discharging body, through conductors or even insulators, but insulators whose surface is covered with a layer that conducts electricity, for example . the surface is wet or dirty. In these cases it happens full R. of a given body, and the duration of this R. is determined by the resistance and shape (see Self-induction) of the conductors through which R. occurs. The lower the resistance and self-induction coefficient of the conductors, the faster R. of the body occurs. The body is partially discharged, i.e. its R. occurs incomplete, when it is connected by conductors to some other body that is not electrified or less electrified than it. In these cases, the more electricity is lost by the body, the greater the capacity of the body that is connected to it through conductors. The phenomena that accompany radiation in the form of current are qualitatively the same as the phenomena that are caused by electric current excited by ordinary galvanic elements. R. conventional occurs when a well-insulated body is in a liquid or gaseous medium containing particles that can become electrified and, under the influence of electrical forces, can move in this medium. R. explosive - this is the R. of the body either into the ground, or into another body, oppositely electrified, through a medium that does not conduct electricity. The phenomenon occurs as if the non-conducting medium yields to the action of those tensions that arise in it under the influence of the electrification of the body, and provides a path for electricity. Such discontinuous R. is always accompanied by light phenomena and can occur in various forms. But all these forms of discontinuous R. can be divided into three categories: R. with the help of a spark, R. using a brush, R. accompanied by radiance, or quiet P. All these R. are similar to each other in that, despite the short duration, each of them represents a combination of several R., that is, with these R. the body does not lose its electricity continuously, but in an intermittent manner. R. with the help of a spark is in most cases oscillatory (see Oscillatory R.). R. with the help of a spark is formed when an electrified body located in some gas considerable elasticity or in a liquid, another body is close enough, conducting electricity and connected to the ground or electrified opposite to this body. A spark can also form when there is a layer of some kind of solid insulator between such two bodies. In this case, the spark pierces this layer, forming a through hole and cracks in it. A spark is always accompanied by a special crackling sound, resulting from a rapid shock to the environment in which it is produced. When the spark is short, it looks like a light, straight line. The thickness of this line is determined by the amount of electricity that is lost by the electrified body with the help of this spark. As the length of the spark increases, it becomes thinner and at the same time deviates from the appearance of a straight line, takes the form of a zigzag line, and then, with further elongation, branches and finally turns into the shape of a brush (Table, Fig. 1). With the help of a rotating mirror, it can be discovered that the spark that appears consists in fact of a number of individual sparks, following one after another after a certain period of time. The length of the resulting spark, or the so-called bit distance, depends on the potential difference between the bodies between which this spark is produced. However, even with the same potential difference between two bodies, the length of the spark formed between them varies somewhat depending on the shape of these bodies. Thus, for a given potential difference, the spark is longer when it is formed between two disks than in the case when it must jump between two balls. And for different balls the spark is not the same length. The more the two balls differ in size, the longer it is. At a given potential difference, the shortest spark is obtained, i.e., the smallest discharge distance is obtained in the case when the spark should be obtained between two balls of the same size. A change in gas elasticity has a very large influence on the magnitude of the potential difference required to form a spark of a given length. As the gas elasticity decreases, this potential difference also decreases. The nature of the gas in which the spark occurs has a significant influence on the magnitude of the required potential difference. For the same spark length and the same gas elasticity, this potential difference is the smallest for hydrogen, it is greater for air and even greater for carbonic acid. To produce a spark in a liquid, a greater potential difference is required than to produce the same spark in a gas. The substance of the bodies between which the spark is formed has a very small effect on the potential difference required for the spark to occur. For short spark lengths in air or any other gas, the potential difference that forms the spark is very closely proportional to the length of the spark. For large spark lengths, the relationship between the spark length and the potential difference required for this is not so simple. In this case, as the potential difference increases, the spark length increases faster than the potential difference increases. The following table contains data for expressing the length of the sparks and the corresponding potential differences (sparks are formed between two disks, one has a slightly convex surface).
| Spark length, in stm | Potential difference, in volts |
| 0,0205 | 1000 |
| 0,0430 | 2000 |
| 0,0660 | 3000 |
| 0,1176 | 5000 |
| 0,2863 | 10000 |
| 0,3378 | 11300 |
ELECTRICAL DISCHARGE.
Under normal conditions, any gas, be it air or silver vapor, is an insulator. In order for a current to arise under the influence of an electric field, the gas molecules must be ionized in some way. The external manifestations and characteristics of discharges in gas are extremely diverse, which is explained by a wide range of parameters and elementary processes that determine the passage of current through the gas. The first includes the composition and pressure of the gas, the geometric configuration of the discharge space, the frequency of the external electric field, current strength, etc., the second includes the ionization and excitation of gas atoms and molecules, recombination impacts of the second kind, elastic scattering of charge carriers, various types of emission electrons. Such a variety of controllable factors creates the prerequisites for a very wide use of gas discharges.
Ionization potential is the energy required to remove an electron from an atom or ion.
Photoionization of atoms. Atoms can become ionized by absorbing light quanta whose energy is equal to or greater than the ionization potential of the atom.
Surface ionization. An adsorbed atom can leave the heated surface in both the atomic and ionized states. For ionization, it is necessary that the work function from the surface be greater than the ionization energy of the level of the valence electron of the adsorbed atom (alkali metals on tungsten and platinum).
Ionization processes are used not only to excite various types of gas discharges, but also to intensify various chemical reactions and to control gas flows using electric and magnetic fields.
A.S. N 444818: A method for heating steel in an oxidizing atmosphere, characterized in that in order to reduce decarbonization, ionized atmospheres are used during the heating process.
A.S. 282684: A method for measuring small flows of gas released into a vacuum volume, characterized in that, in order to increase the measurement accuracy, the gas is ionized before launch and formed into a homogeneous full beam, and then the ion beam is introduced into the vacuum volume, where it is neutralized on a metal target, and the magnitude of the gas flow is judged from the ion beam current.
Typically, a gas discharge occurs between conducting electrodes, which create a boundary configuration of the electric field and play a significant role as sources and sinks of charged particles. However, the presence of electrodes is not necessary (high-frequency toroidal charge).
At sufficiently high pressures and discharge gap lengths, the gaseous medium plays the main role in the occurrence and progression of the discharge. Maintaining the discharge current is determined by maintaining equilibrium gas ionization, which occurs at low currents due to cascade ionization processes, and at high currents due to thermal ionization.
As the gas pressure and the length of the discharge gap decrease, processes on the electrodes play an increasingly important role. At P =0.02..0.4 mmHg/cm, processes on the electrodes become decisive.
At low discharge currents between cold electrodes and a fairly uniform field, the main type of discharge is a glow discharge, characterized by a significant (50 - 400 V) cathode potential drop. The cathode in this type of discharge emits electrons under the influence of charged particles and light quanta, and thermal phenomena do not play a role in maintaining the discharge.
US Patent 3,533,434: A device for reading information from a perforated medium uses glow discharge lamps, which are inexpensive and also highly reliable. Illumination of the lamps through the perforations of the information carrier with a source of pulsating light causes the ignition of some of them, which continues after the disappearance of the light pulse. Thus, glow discharge lamps provide information storage and do not require an additional storage device.
The admixture of molecular gases in the discharge gap during a corona discharge leads to the formation of striations, i.e. dark and light stripes located across the electric field gradient.
A glow discharge in a highly inhomogeneous electric field and significant (P> 100 mmHg) pressure is called a corona discharge. The corona discharge current has the character of pulses caused by electron avalanches. The frequency of occurrence of pulses is 10-100 kHz.
An arc discharge is observed at a current strength of at least several amperes. This type of discharge is characterized by a low (up to 10 V) cathode potential drop and high current density. For an arc discharge, high electron emission from the cathode and thermal ionization in the plasma column are essential. The arc spectrum usually contains lines of cathode material.
A.s. 226 729: A method for rectifying alternating current using a gas-discharge gap with a hollow cathode at low gas pressure corresponding to the region of the left branch of the Paschen curve, characterized in that in order to increase the rectified current and reduce the voltage drop during the conducting part of the period, with a positive potential at the anode transfer the anode-hollow cathode system to arc discharge mode.
A spark discharge begins with the formation of streamers - self-propagating electron avalanches that form a conducting channel between the electrodes. The second stage of the spark discharge - the main discharge - occurs along the channel formed by the streamer, and its characteristics are close to an arc discharge, limited in time by the capacitance of the electrodes and insufficient power supply. At a pressure of 1 atm. the material and condition of the electrodes does not affect the breakdown voltage in this type of discharge.
The distance between the spherical electrodes, corresponding to the occurrence of spark breakdown, is very often used to measure high voltages.
A.s. 272 663: A method for determining the size of macroparticles by applying them to a charged surface, characterized in that, in order to increase the accuracy of the measurement, the intensity of the light flash accompanying the electrical breakdown between the charged surface and the particle approaching it is determined, and the size of the particle is judged by the intensity.
Torch discharge is a special type of high-frequency single-electrode discharge. At pressures close to or above atmospheric pressure, the torch discharge has the shape of a candle flame. This type of discharge can exist at frequencies of 10 MHz, provided the source power is sufficient.
When studying a charged tip, an interesting effect is observed - the so-called flow of charges from the tip. In reality there is no runoff. The mechanism of this phenomenon is as follows: small amounts of free charges in the air near the tip are accelerated and, hitting gas atoms, ionize them. A region of space charge is created, from where ions of the same sign as the tip are pushed out by the field, dragging gas atoms with them. The flow of atoms and ions creates the impression of charges flowing down. In this case, the tip is discharged and at the same time receives an impulse directed against the tip.
Several examples of the use of corona discharge:
A.s. 485 282: An air conditioning device containing a housing with a tray and pipes for air supply and exhaust and a heat exchanger located in the housing with channels irrigated from one of the flows, characterized in that, in order to increase the degree of air cooling by intensifying evaporation, corona water , along the axis of the irrigated channels of the heat exchanger, electrodes are installed, attached to a grounded body using insulators and connected to the negative pole of the voltage source.
A.S. 744429: Corona discharge gauge for wire diameters finer than fifty microns. As is known, a corona discharge in the form of a luminous ring appears around a conductor if a high voltage is applied to the conductor. When determining the cross-section of the conductor, the corona discharge will have very specific characteristics. As soon as the cross section is changed, the characteristics of the corona discharge immediately change.
The century in which we live can be called the time of electricity. The operation of computers, televisions, cars, satellites, artificial lighting devices is just a small part of the examples where it is used. One of the interesting and important processes for humans is electrical discharge. Let's take a closer look at what it is.
A Brief History of the Study of Electricity
When did man become familiar with electricity? It is difficult to answer this question, since it is posed incorrectly, because the most striking natural phenomenon is lightning, known since time immemorial.
The meaningful study of electrical processes began only at the end of the first half of the 18th century. Here it is worth noting the serious contribution to human ideas about electricity by Charles Coulomb, who studied the force of interaction of charged particles, Georg Ohm, who mathematically described the parameters of current in a closed circuit, and Benjamin Franklin, who conducted many experiments studying the nature of the above-mentioned lightning. In addition to them, scientists such as Luigi Galvani (study of nerve impulses, invention of the first “battery”) and Michael Faraday (study of current in electrolytes) played a major role in the development.
The achievements of all these scientists have created a solid foundation for the study and understanding of complex electrical processes, one of which is electric discharge.
What is a discharge and what conditions are necessary for its existence?
Electric current discharge is a physical process that is characterized by the presence of a flow of charged particles between two spatial regions that have different potentials in a gaseous environment. Let's look at this definition.
Firstly, when they talk about discharge, they always mean gas. Discharges in liquids and solids can also occur (breakdown of a solid capacitor), but the process of studying this phenomenon is easier to consider in a less dense medium. Moreover, it is discharges in gases that are often observed and are of great importance for human life.
Secondly, as stated in the definition of an electrical discharge, it occurs only when two important conditions are met:
- when there is a potential difference (electric field strength);
- presence of charge carriers (free ions and electrons).
The potential difference ensures the directional movement of the charge. If it exceeds a certain threshold value, then the non-self-sustaining discharge becomes self-sustaining or independent.
As for free charge carriers, they are always present in any gas. Their concentration, naturally, depends on a number of external factors and the properties of the gas itself, but the very fact of their presence is indisputable. This is due to the existence of such sources of ionization of neutral atoms and molecules, such as ultraviolet rays from the Sun, cosmic radiation and natural radiation of our planet.
The relationship between the potential difference and carrier concentration determines the nature of the discharge.
Types of electrical discharges
We provide a list of these types, and then describe each of them in more detail. So, all discharges in gaseous media are usually divided into the following:
- smoldering;
- spark;
- arc;
- crown.
Physically, they differ from each other only in power (current density) and, as a consequence, temperature, as well as the nature of their manifestation over time. In all cases, we are talking about the transfer of a positive charge (cations) to the cathode (low potential area) and a negative charge (anions, electrons) to the anode (high potential area).
Glow discharge
For its existence it is necessary to create low gas pressures (hundreds and thousands of times less than atmospheric pressure). A glow discharge is observed in cathode tubes that are filled with some gas (for example, Ne, Ar, Kr and others). The application of voltage to the electrodes of the tube leads to the activation of the following process: the cations present in the gas begin to move rapidly, reaching the cathode, they strike it, transmitting an impulse and knocking out electrons. The latter, in the presence of sufficient kinetic energy, can lead to the ionization of neutral gas molecules. The described process will be self-sustaining only if there is sufficient energy of cations bombarding the cathode and a certain amount of them, which depends on the potential difference across the electrodes and the gas pressure in the tube.
The glow discharge glows. The emission of electromagnetic waves is caused by two parallel processes:
- recombination of electron-cation pairs, accompanied by the release of energy;
- transition of neutral gas molecules (atoms) from an excited state to a ground state.
Typical characteristics of this type of discharge are low currents (several milliamps) and low steady-state voltages (100-400 V), but the threshold voltage is several thousand volts, which depends on the gas pressure.
Examples of glow discharge are fluorescent and neon lamps. In nature, this type includes the northern lights (the movement of ion flows in the Earth’s magnetic field).
Spark discharge
This is a typical type of discharge, which manifests itself in For its existence, it is necessary not only the presence of high gas pressures (1 atm or more), but also enormous voltages. Air is a fairly good dielectric (insulator). Its permeability ranges from 4 to 30 kV/cm, which depends on the presence of moisture and solid particles. These figures indicate that to obtain a breakdown (spark) it is necessary to apply at least 4,000,000 volts per meter of air!
In nature, such conditions arise in cumulus clouds when, as a result of the processes of friction between air masses, air convection and crystallization (condensation), charges are redistributed in such a way that the lower layers of clouds are charged negatively, and the upper layers are charged positively. The potential difference gradually accumulates, and when its value begins to exceed the insulating capabilities of air (several million volts per meter), lightning occurs - an electrical discharge that lasts for a fraction of a second. The current strength in it reaches 10-40 thousand amperes, and the plasma temperature in the channel rises to 20,000 K.
The minimum energy that is released in the lightning process can be calculated if we take into account the following data: the process develops during t=1*10 -6 s, I = 10,000 A, U = 10 9 V, then we get:
E = I*U*t = 10 million J
The resulting figure is equivalent to the energy that is released during the explosion of 250 kg of dynamite.
Just like spark, it occurs when there is sufficient pressure in the gas. Its characteristics are almost completely similar to the spark one, but there are also differences:
- firstly, currents reach ten thousand amperes, but the voltage is several hundred volts, which is due to the high conductivity of the medium;
- secondly, an arc discharge exists stable over time, unlike a spark discharge.
The transition to this type of discharge is carried out by a gradual increase in voltage. The discharge is maintained due to thermionic emission from the cathode. A striking example of this is the welding arc.
Corona discharge
This type of electrical discharge in gases was often observed by sailors who traveled to the New World discovered by Columbus. They called the bluish glow at the ends of the masts "St. Elmo's lights."
A corona discharge occurs around objects that have a very strong electric field strength. Such conditions are created near sharp objects (ship masts, buildings with pointed roofs). When a body has some static charge, the field strength at its ends leads to ionization of the surrounding air. The resulting ions begin their drift towards the field source. These weak currents, causing similar processes as in the case of a glow discharge, lead to the appearance of a glow.
Danger of discharges to human health
Corona and glow discharges do not pose a particular danger to humans, since they are characterized by low currents (milliamps). The other two discharges mentioned above are lethal in case of direct contact with them.
If a person observes the approach of lightning, then he should turn off all electrical appliances (including mobile phones), and also position himself so as not to stand out from the surrounding area in terms of height.
Electric discharge- the process of flow of electric current associated with a significant increase in the electrical conductivity of the medium relative to its normal state.
The increase in electrical conductivity is ensured by the presence of additional free charge carriers. Electrical discharges can be non-self-sustaining, occurring due to an external source of free charge carriers, and independent, continuing to burn even after the external source of free charge carriers is turned off.
The following types of electrical discharges are distinguished: spark, corona, arc (electric arc) and glow.
Let's connect the ball electrodes to the battery of capacitors and start charging the capacitors using an electric machine. As the capacitors charge, the potential difference between the electrodes will increase, and consequently, the field strength in the gas will increase. As long as the field strength is low, no changes can be noticed in the gas. However, with sufficient field strength (about 30,000 V/cm), an electric spark appears between the electrodes, which looks like a brightly glowing winding channel connecting both electrodes. The gas near the spark heats up to a high temperature and suddenly expands, causing sound waves to appear and we hear a characteristic crackling sound. Capacitors in this setup are added to make the spark more powerful and therefore more effective.
The described form of gas discharge is called spark discharge, or gas spark breakdown. When a spark discharge occurs, the gas suddenly, abruptly, loses its insulating properties and becomes a good conductor. The field strength at which gas spark breakdown occurs has a different value for different gases and depends on their state (pressure, temperature). For a given voltage between the electrodes, the field strength is lower, the further the electrodes are from each other. Therefore, the greater the distance between the electrodes, the greater the voltage between them is necessary for spark breakdown of the gas to occur. This voltage is called breakdown voltage.
The occurrence of a breakdown is explained as follows. There is always a certain amount of ions and electrons in a gas, arising from random causes. Usually, however, their number is so small that the gas practically does not conduct electricity. At relatively small values of field strength, such as we encounter when studying non-self-sustaining conductivity of gases, collisions of ions moving in an electric field with neutral gas molecules occur in the same way as collisions of elastic balls. With each collision, the moving particle transfers part of its kinetic energy to the resting one, and both particles fly apart after the impact, but no internal changes occur in them. However, if the field strength is sufficient, the kinetic energy accumulated by the ion in the interval between two collisions can become sufficient to ionize the neutral molecule upon collision. As a result, a new negative electron and a positively charged residue - an ion - are formed. This ionization process is called impact ionization, and the work that needs to be expended to remove an electron from an atom is called ionization work. The amount of ionization work depends on the structure of the atom and is therefore different for different gases.
Electrons and ions formed under the influence of impact ionization increase the number of charges in the gas, and in turn they come into motion under the influence of an electric field and can produce impact ionization of new atoms. Thus, this process “reinforces itself”, and ionization in the gas quickly reaches a very large value. All phenomena are quite similar to a snow avalanche in the mountains, for the occurrence of which an insignificant lump of snow is enough. Therefore, the described process was called an ion avalanche. The formation of an ion avalanche is the process of spark breakdown, and the minimum voltage at which an ion avalanche occurs is the breakdown voltage. We see that during a spark breakdown the reason for gas ionization is the destruction of atoms and molecules during collisions with ions.
One of the natural representatives of a spark discharge is lightning - beautiful and not safe.
The occurrence of an ion avalanche does not always lead to a spark, but can also cause a discharge of another type - a corona discharge.
Let's stretch a metal wire AB with a diameter of a few tenths of a millimeter on two high insulating supports and connect it to the negative pole of a generator that provides a voltage of several thousand volts, for example, to a good electric machine. We will take the second pole of the generator to the Earth. We will get a kind of capacitor, the plates of which are our wire and the walls of the room, which, of course, communicate with the Earth. The field in this capacitor is very inhomogeneous, and its intensity is very high near a thin wire. By gradually increasing the voltage and observing the wire in the dark, you can notice that at a certain voltage, a faint glow (“corona”) appears near the wire, covering the wire on all sides; it is accompanied by a hissing sound and a slight crackling sound. If a sensitive galvanometer is connected between the wire and the source, then with the appearance of a glow, the galvanometer shows a noticeable current flowing from the generator through the wires to the wire and from it through the air of the room to the walls connected to the other pole of the generator. The current in the air between the wire AB and the walls is carried by ions formed in the air due to impact ionization. Thus, the glow of air and the appearance of current indicate strong ionization of air under the influence of an electric field.
Corona discharge can occur not only at the wire, but also at the tip and in general at all electrodes, near which a very strong inhomogeneous field is formed.
Application of corona discharge.
1) Electrical gas purification (electric precipitators). A vessel filled with smoke suddenly becomes completely transparent when sharp metal electrodes connected to an electrical machine are inserted into it. Inside the glass tube there are two electrodes: a metal cylinder and a thin metal wire hanging along its axis. The electrodes are connected to the electrical machine. If a stream of smoke (or dust) is blown through a tube and the machine is operated, then as soon as the voltage becomes sufficient to form a corona, the escaping stream of air will become completely clean and transparent, and all solid and liquid particles contained in the gas will be deposited on electrodes. The explanation of the experience is as follows. Once the wire's corona is ignited, the air inside the tube becomes highly ionized. Gas ions, colliding with dust particles, “stick” to the latter and charge them. Since there is a strong electric field inside the tube, charged particles move under the influence of the field to the electrodes, where they settle. The described phenomenon is currently finding technical application for purifying industrial gases in large volumes from solid and liquid impurities.
2) Elementary particle counters. Corona discharge underlies the operation of extremely important physical devices: the so-called counters of elementary particles (electrons, as well as other elementary particles that are formed during radioactive transformations) Geiger-Müller counter. It consists of a small metal cylinder A, equipped with a window, and a thin metal wire stretched along the axis of the cylinder and insulated from it. The meter is connected to a circuit containing a voltage source B of several thousand volts. The voltage is chosen so that it is only slightly less than the “critical” one, i.e., necessary to ignite the corona discharge inside the meter. When a fast-moving electron enters the counter, the latter ionizes the gas molecules inside the counter, causing the voltage required to ignite the corona to slightly decrease. A discharge occurs in the meter, and a weak short-term current appears in the circuit.
The current arising in the meter is so weak that it is difficult to detect with a conventional galvanometer. However, it can be made quite noticeable if a very large resistance R is introduced into the circuit and a sensitive electrometer E is connected in parallel to it. When a current I appears in the circuit, a voltage U is created at the ends of the resistance, equal to Ohm’s law U = IxR. If you choose a resistance value R very large (many millions of ohms), but significantly less than the resistance of the electrometer itself, then even a very weak current will cause a noticeable voltage. Therefore, every time a fast electron gets inside the counter, the electrometer leaf will give off.
Such counters make it possible to register not only fast electrons, but also, in general, any charged, rapidly moving particles capable of ionizing a gas through collisions. Modern counters easily detect the entry of even one particle into them and, therefore, make it possible to verify with complete reliability and very clear clarity that elementary particles really exist in nature.
In 1802, V.V. Petrov established that if you attach two pieces of charcoal to the poles of a large electrolytic battery and, bringing the coals into contact, slightly separate them, a bright flame will form between the ends of the coals, and the ends of the coals themselves will become white hot. Emitting a blinding light ( electric arc). This phenomenon was independently observed seven years later by the English chemist Davy, who proposed calling this arc “voltaic” in honor of Volta.
Typically, the lighting network is powered by alternating current. The arc, however, burns more steadily if a constant current is passed through it, so that one of its electrodes is always positive (anode) and the other negative (cathode). Between the electrodes there is a column of hot gas that conducts electricity well. In ordinary arcs, this pillar emits significantly less light than hot coals. Positive coal, having a higher temperature, burns faster than negative coal. Due to the strong sublimation of coal, a depression is formed on it - a positive crater, which is the hottest part of the electrodes. The temperature of the crater in air at atmospheric pressure reaches 4000 °C. The arc can also burn between metal electrodes (iron, copper, etc.). In this case, the electrodes melt and quickly evaporate, which consumes a lot of heat. Therefore, the crater temperature of a metal electrode is usually lower than that of a carbon electrode (2000-2500 °C).
By forcing an arc to burn between carbon electrodes in compressed gas (about 20 atm), it was possible to bring the temperature of the positive crater to 5900 °C, i.e., to the temperature of the surface of the Sun. Under this condition, coal melting was observed.
The column of gases and vapors through which the electric discharge occurs has an even higher temperature. The energetic bombardment of these gases and vapors by electrons and ions, driven by the electric field of the arc, brings the temperature of the gases in the column to 6000-7000 °. Therefore, in the arc column, almost all known substances melt and turn into steam, and many chemical reactions are made possible that do not occur at lower temperatures. It is not difficult, for example, to melt refractory porcelain sticks in an arc flame. To maintain an arc discharge, a small voltage is needed: the arc burns well when the voltage at its electrodes is 40-45 V. The arc current is quite significant. So, for example, even in a small arc, a current of about 5 A flows, and in large arcs used in industry, the current reaches hundreds of amperes. This shows that the arc resistance is low; Consequently, a luminous gas column conducts electric current well.
Such strong ionization of the gas is possible only due to the fact that the arc cathode emits a lot of electrons, which, with their impacts, ionize the gas in the discharge space. Strong electron emission from the cathode is ensured by the fact that the arc cathode itself is heated to a very high temperature (from 2200° to 3500°C depending on the material). When, to ignite an arc, we first bring the coals into contact, then at the point of contact, which has a very high resistance, almost all the Joule heat of the current passing through the coals is released. Therefore, the ends of the coals become very hot, and this is enough for an arc to break out between them when they move apart. Subsequently, the cathode of the arc is maintained in a heated state by the current itself passing through the arc. The main role in this is played by the bombardment of the cathode by positive ions incident on it.
Application of arc discharge.
Due to the high temperature, the arc electrodes emit dazzling light, and therefore the electric arc is one of the best light sources. It consumes only about 0.3 watts per candle and is significantly more economical. Than the best incandescent lamps. The electric arc was first used for lighting by P. N. Yablochkov in 1875 and was called the “Russian light”, or “northern light”.
The electric arc is also used for welding metal parts (electric arc welding). Currently, the electric arc is very widely used in industrial electric furnaces. In global industry, about 90% of tool steel and almost all special steels are smelted in electric furnaces.
Of great interest is a mercury arc burning in a quartz tube, the so-called quartz lamp. In this lamp, the arc discharge occurs not in the air, but in an atmosphere of mercury vapor, for which a small amount of mercury is introduced into the lamp, and the air is pumped out. Mercury arc light is extremely rich in invisible ultraviolet rays, which have strong chemical and physiological effects. Mercury lamps are widely used in the treatment of various diseases (“artificial mountain sun”), as well as in scientific research as a strong source of ultraviolet rays.
In addition to the spark, corona and arc, there is another form of independent discharge in gases - the so-called glow discharge. To obtain this type of discharge, it is convenient to use a glass tube about half a meter long, containing two metal electrodes. Let's connect the electrodes to a direct current source with a voltage of several thousand volts (an electric machine will do) and gradually pump out the air from the tube. At atmospheric pressure, the gas inside the tube remains dark because the applied voltage of several thousand volts is not enough to pierce the long gas gap. However, when the gas pressure drops sufficiently, a luminous discharge flashes in the tube. It looks like a thin cord (crimson in air, other colors in other gases) connecting both electrodes. In this state, the gas column conducts electricity well.
With further evacuation, the luminous cord blurs and expands, and the glow fills almost the entire tube. The following two parts of the discharge are distinguished: 1) the non-luminous part adjacent to the cathode, called the dark cathode space; 2) a luminous column of gas filling the rest of the tube, right up to the anode. This part of the discharge is called the positive column.
During a glow discharge, gas conducts electricity well, which means that strong ionization is maintained in the gas all the time. In this case, unlike an arc discharge, the cathode remains cold all the time. Why does the formation of ions occur in this case?
The drop in potential or voltage for each centimeter of length of the gas column in a glow discharge is very different in different parts of the discharge. It turns out that almost the entire drop in potential occurs in dark space. The potential difference that exists between the cathode and the space boundary closest to it is called the cathode potential drop. It is measured in hundreds, and in some cases thousands of volts. The entire discharge appears to exist due to this cathode fall. The significance of the cathode fall is that positive ions, running through this large potential difference, acquire greater speed. Since the cathode incidence is concentrated in a thin layer of gas, almost no collisions of ions with gas atoms occur here, and therefore, passing through the cathode incidence region, the ions acquire very high kinetic energy. As a result, when they collide with the cathode, they knock out a certain number of electrons from it, which begin to move towards the anode. Passing through dark space, electrons, in turn, are accelerated by the cathode potential drop and, when colliding with gas atoms in a more distant part of the discharge, produce impact ionization. The positive ions that arise in this case are again accelerated by the cathode fall and knock out new electrons from the cathode, etc. Thus, everything is repeated as long as there is voltage on the electrodes.
This means that the reasons for gas ionization in a glow discharge are impact ionization and knocking out electrons from the cathode by positive ions.
Application of glow discharge.
This discharge is used mainly for lighting. Used in fluorescent lamps.
Electrical discharge*- The loss of electricity by any electrified body, i.e. the electric discharge* of this body, can occur in various ways, as a result of which the phenomena accompanying the electric discharge* can be very different in nature. All the various forms of electrical discharge* can be divided into three main types: Electrical discharge* in the form of electric current, or Electrical* conductive discharge, Electrical* convective discharge and Electrical* explosive discharge. Electrical discharge* in the form of current occurs when an electrified body is connected to the earth or to another body possessing m, equal in quantity and opposite in sign to the electricity on the discharging body, through conductors or even insulators, but insulators that are covered with a layer that conducts electricity, for example. the surface is wet or dirty. In these cases it happens full discharge electrical* of a given body, and the duration of this electric discharge * is determined by the m and shape (see) of the conductors through which the electric discharge occurs * The lower the resistance and self-induction of the conductors, the faster the electric discharge * of the body occurs. The body is partially discharged, i.e. its electrical discharge* occurs incomplete, when it is connected by conductors to some other body that is not electrified or less electrified than it. In these cases, the more electricity is lost by the body, the greater the capacity of the body that is connected to it through conductors. The phenomena accompanying an electric discharge* in the form of a current are qualitatively the same as the phenomena that are caused by an electric current excited by ordinary galvanic elements. Electrical discharge* conventional occurs when a well-insulated body is in a liquid or gaseous medium containing particles that can become electrified and, under the influence of electrical forces, can move in this medium. Electrical discharge* explosive- this is an electric discharge* of a body either into the ground or into another body, oppositely electrified, through a medium that does not conduct electricity. The phenomenon occurs as if the non-conducting medium yields to the action of those tensions that arise in it under the influence of the electrification of the body, and provides a path for electricity. Such an explosive electrical discharge* is always accompanied by light phenomena and can occur in various forms. But all these forms of explosive electrical discharge* can be divided into three categories: Electrical discharge* with the help of a spark, Electrical discharge* using a brush, Electrical discharge* accompanied by radiance, or quiet P. All of these electrical discharges* are similar to each other in that, despite the short duration, each of them represents a combination of several electrical discharges*, i.e., with these electrical discharges* the body loses its electricity not continuously, but intermittently way. Electrical discharge* with the help of a spark is in most cases oscillatory (see Oscillatory Electrical Discharge*). An electric discharge* with the help of a spark is formed when an electrified body located in any gas considerable elasticity or in a liquid, another body is close enough, conducting electricity and connected to the ground or electrified opposite to this body. A spark can also form when there is a layer of some kind of solid insulator between such two bodies. In this case, the spark pierces this layer, forming a through hole and cracks in it. A spark is always accompanied by a special crackling sound, resulting from a rapid shock to the environment in which it is produced. When the spark is short, it looks like a light, straight line. The thickness of this line is determined by the amount of electricity that is lost by the electrified body with the help of this spark. As the length of the spark increases, it becomes thinner and at the same time deviates from the appearance of a straight line, takes the form of a zigzag line, and then, with further elongation, branches and finally turns into the shape of a brush (Table, Fig. 1). With the help of a rotating mirror, one can discover that the spark that appears is actually made up of a number of individual sparks, following one after another after some time. The length of the resulting spark, or the so-called bit distance, depends on the potential difference between the bodies between which this spark is produced. However, even with the same potential difference between two bodies, the length of the spark formed between them varies somewhat depending on the shape of these bodies. Thus, for a given potential difference, the spark is longer when it is formed between two disks than in the case when it must jump between two balls. And for different balls the spark is not the same length. The more the two balls differ in size, the longer it is. At a given potential difference, the shortest spark is obtained, i.e., the smallest discharge distance is obtained in the case when the spark should be obtained between two balls of the same size. A change in gas elasticity has a very large influence on the magnitude of the potential difference required to form a spark of a given length. As the gas elasticity decreases, this potential difference also decreases. The gas in which the spark occurs has a significant influence on the magnitude of the required potential difference. For the same spark length and the same gas elasticity, this potential difference is the smallest for hydrogen, it is greater for air and even greater for carbonic acid. To produce a spark in a liquid, a greater potential difference is required than to produce the same spark in a gas. The substance of the bodies between which the spark is formed has a very small effect on the potential difference required for the spark to occur. For short spark lengths in air or any other gas, the potential difference that forms the spark is very closely proportional to the length of the spark. For large spark lengths, the relationship between the spark length and the potential difference required for this is not so simple. In this case, as the potential difference increases, the spark length increases faster than the potential difference increases. The following table contains data for expressing the length of the sparks and the corresponding potential differences (sparks are formed between two disks, one has a slightly convex surface).
| Spark length, in stm | Potential difference, in volts |
ELECTRICAL DISCHARGE.